I've just had a bike in with exactly the same problem with some Shimano RSX's. No positive clicking up or down from either lever although sometimes they would randomly work.
A lot of STI's are sealed units and although I've heard of people taking them apart, they're near impossible to get back together. Because their sealed the grease clogs up and dries out inside.
The trick is to turn the bike upside down, get your WD40 can with its straw, push the straw up 'between the back of the brake lever and front the small shifter' and flood it with WD. This is the only point where you can get the WD40 inside the shifter mechanism.
Leave it over night and hopefully next day you'll have perfect shifting